Potential Vorticity Diagnosis of a Tropopause Polar Cyclone

Abstract

Long-lived coherent vortices located near the tropopause are often found over polar regions. Although these vortices are a commonly observed feature of the Arctic, and can have lifetimes longer than one month, little is known about the mechanisms that control their evolution. This paper examines mechanisms of intensity change for a cyclonic tropopause polar vortex (TPV) using an Ertel potential vorticity (EPV) diagnostic framework. Results from a climatology of intensifying cyclonic TPVs suggest that the essential dynamics are local to the vortex, rather than a consequence of larger-scale processes. This fact motivates a case study using a numerical model to investigate the role of diabatic mechanisms in the growth and decay of a particular cyclonic vortex. A component-wise breakdown of EPV reveals that cloud-top radiational cooling is the primary diabatic mechanism that intensifies the TPV during the growth phase. Increasing amounts of moisture become entrained into the vortex core at later times near Hudson Bay, allowing the destruction of potential vorticity near the tropopause due to latent heat release to become comparable to the radiational tendency to create potential vorticity.1 Assuming adiabatic and inviscid flow, PV and potential temperature are both conserved following the fluid motion. Thus, the existence of closed PV contours on potential temperature surfaces implies the existence of closed contours of potential temperature on PV surfaces.

Radiative Impact on Tropopause Polar Vortices over the Arctic

“…Cyclonic TPVs are frequently observed, high-latitude, cold-core vortices based on the tropopause and play an important role in the formation of surface cyclones. While radiative cooling may directly promote intensification through changes in moisture near TPVs, latent heating in clouds has a weakening effect; however, clouds can also indirectly promote radiative intensification through cloud-top cooling (Cavallo and Hakim 2009). While radiative cooling may directly promote intensification through changes in moisture near TPVs, latent heating in clouds has a weakening effect; however, clouds can also indirectly promote radiative intensification through cloud-top cooling (Cavallo and Hakim 2009).…”

“…Furthermore, TPVs are better expressed as vortical rather than wave-like features (e.g., Hakim 2000) and, because of the conservation properties of potential vorticity (PV) and potential temperature, are well identified in these fields and can be tracked for long periods of time on a single PV surface (e.g., Morgan and Nielsen-Gammon 1998). Additional studies have identified cyclonic TPVs as having the following characteristics: a downward intrusion of stratospheric air to ;500 hPa (e.g., Uccellini et al 1985;Davis and Emanuel 1991;Hakim 2000;Cavallo and Hakim 2009;CH10), radii up to ;1200 km, amplitudes of up to 50 K (e.g., Hakim 2000;Hakim and Canavan 2005;CH10), and nearly saturated atmospheric conditions near the vortex core (Cavallo and Hakim 2009;CH10). Furthermore, the Canadian Archipelago region has been identified as a preferred location for intensification of cyclonic TPVs (Cavallo and Hakim 2009).…”

“…Additional studies have identified cyclonic TPVs as having the following characteristics: a downward intrusion of stratospheric air to ;500 hPa (e.g., Uccellini et al 1985;Davis and Emanuel 1991;Hakim 2000;Cavallo and Hakim 2009;CH10), radii up to ;1200 km, amplitudes of up to 50 K (e.g., Hakim 2000;Hakim and Canavan 2005;CH10), and nearly saturated atmospheric conditions near the vortex core (Cavallo and Hakim 2009;CH10). Furthermore, the Canadian Archipelago region has been identified as a preferred location for intensification of cyclonic TPVs (Cavallo and Hakim 2009). …”

“…A review of the methods used in Cavallo and Hakim (2009) and CH10 to quantify vortex intensity change is given in section 2. A review of the methods used in Cavallo and Hakim (2009) and CH10 to quantify vortex intensity change is given in section 2.…”

Phases of the Isobaric Surface Shapes in the Geostrophic State of the Atmosphere and Connection to the Polar Vortices

“…The question is raised here as to whether the polar vortex results from the atmosphere geostrophic state disturbance owing to the non-uniform heating of the Earth from the equator to the pole or if it is inherent in the geostrophic state itself. At present, a number of various approaches to study the polar vortex formation and development mechanisms exist [19][20][21][22][23][24][25]. Most of them consider the polar extrema of the pressure field as an effect of the geostrophic state disturbance but not as the geostrophic state feature.…”